An Extra class created by Heartless in order to summon imposters and body doubles.
Within the story of Case Files, it refers to Iskandar’s body double, who ignores the Ionian Hetairoi. Her Personal Skill “He is Another Iskandar (Fake)” grants her some of Iskandar’s Personal Skills, as well as a distorted version of the Noble Phantasm Gordius Wheel, a sight that shook El-Melloi II to his core on their first meeting.
Her true identity is that of Hephaestion’s younger twin sister, born at the behest of the King of Conquerer’s mother Olympias, to protect him for his entire life.
Though it may be hard to imagine her as a body double for a man like Iskandar who was over two meters tall, her role as a body double was a magical one.
As such, she had no name.
After separating from Olympias, Iskandar tried to give her one numerous times. Since doing so would weaken the spells redirecting curses from him to her, however, she rejected each time. Not only that, but for the sake of protecting Iskandar from various forms of curses and magecraft, she developed numerous counter-intelligence schemes. That is why the descriptions left behind about Iskandar don’t match the appearance of the Servant Iskandar at all.
The Class name of Faker is, in a way, the first name she ever received.
As she didn’t leave behind any anecdotal stories in history, her status as a Servant is rather weak, but she still boasts a considerable and comprehensive combat ability. This results not just from having powerful skills or Noble Phantasms, but from the wide variety of physical and magical options available to her in combat, and her ability to keenly recognize which of those options is optimal for each situation.
Just as she said during the story, “Being a warrior is a question of both body, mind, and spirit.” Her love of battle and the skills that saw her through numerous battlefields made it possible for her and Heartless to break through the Spiritual Tomb of Albion alone.
Despite how much of a mismatched pair the two of them may seem, her partnership with Heartless is one she finds unexpectedly comfortable.
Though it was never discussed in detail, the way Heartless’ life changed after being betrayed by his students and her unwillingness to forgive the betrayal she suffered in the Wars of the Diadochi1 demonstrated a clear unity of values betwen them.

The Dionysus that Faker references normally corresponds to the god of wine Bacchus, but the magecraft she uses actually points to the maddened Zeus himself. In short, an omnipotent god gone mad. Abandoning reason and embracing infinite chaos is truly suitable as a source for magecraft.
Of course, as one who serves the god of wine, nevermind Iskandar, she regularly drank all of his close friends and advisors under the table.
In addition, the reason she rejects the summons of the Ionian Hetairoi comes from the following flow of events. “She is called by Iskandar” ⮕ “Immediately before being summoned, she is given the fundamental knowledge from the world about the Wars of the Diadochi” ⮕ “Before the summoning is completed, she rejects it.” A summoned Heroic Spirit, if not from the Holy Grail then from the World itself, receives knowledge about the present era. In the case of Ionian Hetairoi, it is the latter that provides the information.
Were she to be summoned again as a Servant, as usual she would forget everything that happened during the course of these incidents. But what if she were summoned as part of the Army of the King? What if she were summoned in a Singularity that ignored the correct chronology of history, or by a system completely different than the one set up in Fuyuki?
The answer will remain a mystery until such an event occurs.

Translator’s Notes
  1. ^ From Wikipedia: The Wars of the Diadochi, or Wars of Alexander’s Successors, were a series of conflicts fought between Alexander the Great’s generals over the rule of his vast empire after his death. They occurred between 322 and 281 BC.
Lord El-Melloi II Case Files material: Lord El-Melloi II Case Files Glossary